Nucleophilic Aromatic Substitution

Let’s review electrophilic aromatic substitution for a moment.

In electrophilic aromatic substitution:

the aromatic ring is the nucleophile
we add an electrophilic reagent
we form a carbocation intermediate
electron donating groups increase the rate of reaction (since they stabilize the carbocation)
electron withdrawing groups decrease the rate of reaction (they destabilize the carbocation)

There’s a second class of aromatic substitution called nucleophilic aromatic substitution. In this reaction, we’re still forming and breaking a bond on a carbon of the aromatic group, but the polarity of the reagents is completely switched.

In nucleophilic aromatic substitution:

the aromatic ring is the electrophile
we add a nucleophilic reagent
we form a carbanion intermediate
electron donating groups decrease the rate of reaction (they destabilize the carbanion)
electron withdrawing groups increase the rate of reaction (since they stabilize the carbanion)

This is more like the SN2 reaction that you learned in first-semester organic chemistry. Except here we’re not doing a backside attack, we’re attacking the aromatic ring. And then the negative charge that forms can expel the leaving group.

Here’s a comparison of each.

Nucleophilic aromatic substitution (also called addition-elimination), requires an electron withdrawing substituent to be present on the ring ortho or para to the halide being substituted. The reaction is difficult to achieve with aryl halides for which halogen is the lone ring substituent. If a strongly electron-withdrawing substituent is present, however, ortho or para to the halogen, the carbanion intermediate will be more stable. A substituent such as nitro, while deactivating electrophilic aromatic substitution (which forms a carbocation intermediate), activates ortho and para positions, if they contain halogen, for nucleophilic aromatic substitution. Basicity of the nucleophile also facilitates the reaction